In general, it has been long known to prepare thermoplastic granules formed from a mineral and/or organic pulverulent material, such as calcium carbonate, which material is finely ground, said granules may also contain in addition various agents such as stabilizers, lubrifacients, plasticizers, crosslinking agents, biocides, fire retardants, and a polyolefin. The mixture of these various constituents is submitted to thermal and mechanical treatments comprising malaxation and extrusion.
In the past, those skilled in the art have tried to vary the composition of the granules to maximize the amount of mineral and/or organic material, using the approach of employing more and more powerful malaxation devices. It is recognized today that it is difficult to achieve a proportion of mineral matter greater than 75 wt. % of the granules, i.e., 300 parts by weight per 100 parts by weight of polymer, without encountering major difficulties in the preparation and/or use of the granule products, for example, difficulties in the redispersion of the mineral material in the polymers.
One of the major drawbacks which occurs when employing mineral materials in high amounts near the above-mentioned limits is the heterogeneity during the malaxation of the mixture formed from the mineral filler and the polymer, for example, a polyolefin. This heterogeneity may lead to the granules having very heterogeneous compositions, which may be detrimental to their subsequent usefulness. In particular, lack of homogeneity frequently is the cause of the deterioration of mechanical characteristics of plastic materials to which such granules have been added.
Another equally important disadvantage occurs when one skilled in the art attempts to increase to an excessive degree the amount of mineral matter with respect to the amount of thermoplastic polymer employed in the mixture, i.e., when an attempt is made to introduce more than 300 parts by weight of mineral filler per 100 parts by weight of polymer. In this case, at the time of malaxation of the components, and depending on the type of malaxation device employed, either the mixture sets, which causes blocking of the malaxation device (e.g., when said device is a screw-type device), or the constituents separate, making it impossible to combine them using a malaxation device of the type with propellers or rotors.
Further, under these conditions it becomes impossible to use the granules, i.e., to redisperse the heterogeneous compositions into the final thermoplastic polymers, without risking the irregular addition of additives to the plastic materials, whereby the surface appearance and certain material characteristics will be substantially deleteriously affected, and may even be rendered completely unsatisfactory.
Thus, it has been shown that the ratio of the amount of mineral material in the granules to the amount of thermoplastic polymer is naturally limited by the aforesaid major drawbacks.
These difficulties have in the past spurred research into improvements which can be applied to various constituents of the plastic compositions to enable the compositions to contain greater amounts of mineral material without resulting in the said drawbacks.
A number of solutions have been proposed in the literature to alleviate the aforesaid major disadvantages.
First of all, because the mineral materials are frequently hydrophilic in character, they seem to be relatively incompatible with all polymers, and in particular with substantially hydrophobic polymers. It is known that the presence of hydrophilic mineral materials in substantial amounts on the order of those known in the prior art can cause deterioration of the mechanical characteristics of the polymers to which the hydrophilic materials have been added. In order to combat this phenomenon, the literature recommends mitigating the hydrophilic character of the mineral materials (e.g., natural, finely ground calcium carbonates) by treating them with organic substances which form an envelope which is compatible with the polymers. Thus, natural calcium carbonates have been treated with agents chosen from among the unsaturated (or otherwise) carboxylic acids of moderate to high molecular weight, such as butyric, lauric, oleic and stearic acids, and from among the high molecular weight alcohols modified by combination to form, for example, sulfonates or sulfates (Fr. Pat. No. 1,047,087). However, it has been shown that the use of calcium carbonate modified by such a treatment does not enable the quantity of mineral materials in the polymers to be appreciably increased.
Subsequently it was proposed to provide compatibility between the hydrophilic mineral material and the hydrophobic polymer by introducing, during the preparation of the composition, a bonding agent between the mineral filler and the polymer, said agent comprising an organophosphorus compound, namely a phosphonate or a phosphonic acid, such that the plastic molding compound, i.e., the final product, contains from 90-20 parts by weight of a polyolefin, from 10-80 parts by weight of an alkaline earth carbonate, and from 0.1-10 parts by weight of the bonding agent, with the parts by weight being on the basis of the amount of mineral filler (Ger. Pat. No. 2,735,160). However, it was shown that the use of such an agent does not provide the expected improvements. Stated otherwise, the compatibility between the hydrophilic mineral filler and the hydrophobic polyolefins is not appreciably improved, and certain mechanical properties of interest of the filled polymers remain unsatisfactory.
It has also been proposed that the compatibility between the hydrophilic mineral material and the hydrophobic polymer can be improved by substituting, for the above-mentioned bonding agent, an organosulfur compound, namely a sulfate ester, a sulfonic acid, or a derivative of one of these (Eur. Pat. No. 0 017 038). It has been shown that the use of such an organosulfur agent does improve certain mechanical properties of the filled polymers, due to the bonding effect, for certain recommended amounts of mineral material; however, the amount of mineral material remains limited to at most 80 wt. % of the total weight of the composition, i.e., 400 parts by weight of mineral material per 100 parts by weight of the overall sum of the polymer and the bonding agent.
Finally, in U.S. Pat. No. 4,455,344 it has been proposed to prepare granules with the following composition:
(a) 60-90 parts by weight of a mineral filler having a mean dimension of 0.5-100 microns; PA1 (b) 5-35 parts by weight of a crystalline polyolefin having mean dimension of 150-1000 microns; and PA1 (c) 5-35 parts by weight of a binder having a fusion temperature at least 10.degree. C. less than that of the crystalline polyolefin. PA1 (a) 19.99-4.05 units of at least one polyolefin polymer and/or copolymer having a fusion and/or softening temperature of at least 60.degree. C. and an index of fluidity of at least 50; PA1 (b) 80-95 units, preferably 85.7-92.3 units, of pulverulent mineral materials, thus present in the amount of 400-1900 (and preferably 600-1200) parts by weight per 100 parts by weight of the said polyolefin polymer and/or copolymer; PA1 (c) 0.01-0.95 units of an agent which renders the mixture fluid (a fluidifacient).
To yield such granules, the proposed method consists of covering the crystalline polyolefin and/or the particles of mineral filler with the binding agent, to form an envelope which ensures that the particles will mutually adhere. However, it has been experimentally verified that granules prepared according to this process cannot support a concentration of mineral material greater than 80 wt. % without showing poor redispersion into the polymers, even if one employs a mineral material of relatively high mean particle size, e.g., 50 microns. The limit of 80 wt. % becomes absolutely inattainable due to the occurrence of phase separation during the malaxation. This separation occurs when the particle size of the mineral filler is chosen with mean value less than 50 microns, e.g., if a particle mixture is used having mean particle size on the order of 3 microns.
Such a method does not result in a coherent pasty mixture, i.e., a mixture having uniform mixture composition at the temperature at which it is produced and with the means employed. Rather, it yields incoherent agglomerates, i.e., agglomerates which have compositions which generally differ one from the other and are of irregular dimensions, and which later lead to poor redispersion, as the present Applicant has been able to demonstrate.